Clambering onto a dun-colored knoll, not far from the small town of Worland, Wyo., Scott Wing stares out at the deeply abraded hills that sweep towards him like the waves of a vast stony ocean. "That's it," he says, pointing to a sinuous ribbon of rose-colored rock. "That's the Big Red." I follow his gaze, noting how the Big Red snakes into an arroyo, then disappears around a bend. Even to my untrained eye, the geological band seems to glow with a fierce, otherworldly intensity.

In some places, Wing explains, the Big Red is composed of multiple stripes; in others, it wends through the landscape as a single line of color. Then, too, the capricious hand of erosion has exposed it here, left it hidden over there. But after hours of pondering the pieces of this jigsaw, Wing, a paleo-botanist at the Smithsonian's Museum of Natural History in Washington, D.C., believes he can now follow the Big Red for a distance of 25 miles, from the base of the solitary outcrop that looms in the distance all the way to the Sand Creek Divide.

The Sand Creek Divide is a high point in Wyoming's Big Horn Basin. From it you can see the emerald patchwork of irrigated sugar beet and malt barley fields that hug the Big Horn River as well as the jagged mountain ranges — the Absarokas, the Big Horns, the Owl Creeks — that define the edges of this harsh mid-latitude desert. Temperatures here regularly dip below 0 degrees Fahrenheit in wintertime and, in summer, soar well past 100. Away from waterways, the vegetation amounts to little more than a stippling of sagebrush intermingled with stands of invasive cheatgrass and ephemerally blooming wildflowers.

But between 55 and 56 million years ago, says Wing, the Big Horn Basin was a balmy, swampy Eden, teeming with flora and fauna that would be at home in today's coastal Carolinas. Crocodiles, turtles and alligator gar plied the waters of meandering rivers, and early mammals scampered through woodlands filled with the relatives of modern sycamores, bald cypresses and palms. And then, all of a sudden, things got a whole lot warmer. In a geological eye blink — less than 10,000 years, some think — global mean temperatures shot up by around 10 degrees Fahrenheit, jumpstarting a planetary heat wave that lasted for over 150,000 years.

Here, in the southeastern sector of the Basin, the Big Red is the most vivid marker of this exceptionally torrid time — the Paleocene-Eocene Thermal Maximum, or PETM, as most paleontologists call it. By following the Big Red, Wing and his colleagues hope to locate fossils and other clues that will help them reconstruct this long vanished world — a world with unexpected relevance for us as we hurtle towards our own rendezvous with climate change.

Scientists believe that, then as now, the earth warmed in response to a precipitous release of carbon dioxide and other heat-trapping gases, setting in motion events that reverberated through both marine and terrestrial ecosystems. But where did those gases come from so long ago? What triggered their sudden release? And, most important of all, how likely is it that the PETM, or something disquietingly close to it, could happen all over again?

In 1972, when a 17-year-old Wing made the first of many trips to the Big Horn Basin, scientists knew too little even to frame such questions. Today, however, dozens of paleontologists, oceanographers, geochemists and climate modelers are racing to come up with answers. Nowhere have they struck a more productive lode than in these candy-striped badlands. As Wing says, "You can literally walk up to a layer of rock and know that the Paleocene-Eocene boundary starts here."

Leaning on a long-handled shovel, Wing goes over the field schedule with a couple of colleagues, then heads back to Dino, a rust-colored 1970 Suburban with a bird-like dinosaur painted on each side. Wing bought this unlikely chariot in 1987 and somehow has kept it running ever since.

Five minutes later, he pulls up to the site that everyone refers to as "Ross's quarry" in honor of University of Nebraska paleontologist Ross Secord, who discovered it last year. Wing's crew has formed a conga line of shovelers, and as their 53-year-old leader scrambles up from below, they fling clouds of grit in his direction. Eventually, the pace of shoveling slows down so that promising chunks of rock can be individually examined and, if necessary, split open with a hammer. The best specimens are passed to Wing, who peers at each one through his eyepiece and decides whether to keep or discard it.

"This is a good one," Wing calls, so I climb up to see. On the surface of the rock is an exquisitely formed leaf, its veins and margins perfectly preserved. Grayish brown in color and slightly dank, the 55-million-year-old leaf looks like it might have fallen last week and is just now beginning to molder. Adding to the illusion of freshness, its fossilized tissue retains traces of the waxes that once comprised its protective exterior coating.

The plant to which this leaf once belonged, Wing thinks, migrated from far to the south in response to warming temperatures. Like a time capsule, the leaf carries information that can illuminate what it was like to live in a rapidly warming world.

"So far, what we've learned is that processes we're now affecting are so complicated that we can't easily model them," Wing says. "We can monitor them, but over short periods of time there's so much noise in the system that it overwhelms the signal. That's why the geological and paleontological record is so important. It's one of the few ways we can look into how the system works." With that, Wing turns away to squint at another leaf. Unshaven, with a broad-brimmed hat squashed onto his head and a notebook stuffed into a field vest pocket, he looks just like the seasoned fossil hunter he is.